Patch by Francisco Martín Brugué

Remove setting hash seed to regrtest's random seed and re-execv()ing: this doesn't preserve Python flags and fails from a temp directory.

in order to make algorithmic complexity attacks on (e.g.) web apps much more complicated.

The environment variable PYTHONHASHSEED and the new command line flag -R control this
behavior.

Closes #8033.

Fix obscure failures of datetime-related tests due to the datetime tests failing to restore the system state completely after testing the pure-Python versions.

Remove setting hash seed to regrtest's random seed and re-execv()ing: this doesn't preserve Python flags and fails from a temp directory.

Fix "sys.path modified" warning in test_strlit, by not replacing sys.path itself, only its contents.